Nature - USA (2020-05-14)

Nature | Vol 581 | 14 May 2020 | 165

increasing the sensitivity and range compared to time-of-flight lidar
systems, which at present rely on sequential switching of laser diode
arrays. Furthermore, coherent lidar is superior to time-of-flight imple-
mentations in low-visibility and high-background-light conditions,
culminating in achievements such as detecting the distance to objects
engulfed in flames^17 , because delayed homodyne detection makes it
almost impervious to interference and malicious remote attacks^18.
Despite these advantages, coherent ranging suffers from the stringent
requirement of narrow linewidth^5 , as well as fast and linear frequency
chirping^8 , which makes massively parallel implementations, as used
in time-of-flight lidar, challenging.

Concept of soliton-based parallel FMCW ranging

Here we demonstrate a massively parallel coherent FMCW source based
on a soliton microcomb integrated on a photonic chip. Specifically,

we show that agile chirping of the pump laser frequency ωp retains the

soliton state and leads to simultaneous chirping of all comb teeth ω±μ,

where μ denotes the mode number comprising the soliton. The princi-

ples of massively parallel coherent lidar based on soliton microcombs

are illustrated in Fig. 1a. The underlying idea is to transfer the chirp

of a prepared frequency-modulated lidar source to multiple comb

sidebands by using it to generate a dissipative Kerr soliton (DKS)^19 ,^20.

In the time domain (see Fig. 1a), we modulate the underlying soliton

carrier frequency, while minimizing changes of the pulse envelope

and repetition rate. In the frequency domain, this corresponds to a

concurrent modulation of the optical frequency of each comb tooth

around its average value (that is, a modulation of the frequency comb’s

carrier-envelope frequency). This effect, when combined with triangu-

lar frequency modulation of a narrow linewidth pump laser, generates

a massively parallel array of independent FMCW lasers. When dispers-

ing the channels using diffractive optics, as illustrated in Fig. 1b, each

x–μ,v–μ x 0 ,v 0 x+μ,v+μ

CW pump

Time Time

EOM

Z–μ

Z+μ

Zp

Frequency

Principle of FMCW lidar

Time

fbeat fd fd

fu

ΔfD Δt

fu

x B

• μ v–μ

x 0 v 0

DEMUX

x+μ v+μ

Z–μ

Z+μ

(x–μ,v–μ)

(x 0 ,v 0 )

(x+μ,v+μ)

v

b

a

d e

T

t

ΔZ

~sech^2

trep trep

t

Z Z

Si 3 N 4

c

Distance = 4 cTB

fu + fd

2

Velocity =

fu– fd

2 fc cosT 2

c

fopt

T+μ

T 0

90/10

50/50

AFG

Si 3 N 4 CIRC

E(t)

E(Z)

E(t)

E(Z)

Digital signalprocessing

Zp

Fig. 1 | Massively parallel frequency-modulated continuous-wave lidar
using soliton microcombs. a, Principle of DKS generation in a microresonator
with a frequency agile laser. By chirping the continuous-wave (CW) pump laser,
the soliton pulse stream exhibits a change in the underlying carrier, while the
pulse-to-pulse repetition rate 1/trep and sech^2 spectral envelope remains
unchanged. In the frequency domain this corresponds to scanning each
individual comb tooth, that is, a change of the carrier envelope frequency fceo
only. b, Schematic outline of the proposed system design. A frequency
modulated pump laser drives a photonic integrated Si 3 N 4 microresonator. Each
individual sideband, spatially dispersed with diffractive optics, serves as a
source of frequency-modulated laser light in a parallel detection scheme. EOM,
electro-optical phase modulator; AFG, arbitrary function generator; CIRC,

optical circulator; DEMUX, demultiplexer; 50/50 and 90/10, optical splitters.

c, Electron microscope image of 228.43-μm Si 3 N 4 microring resonator.

d, Principle of coherent velocimetry and ranging with multiple optical carriers

isolated from a soliton microcomb. Interleaved upwards and downwards

frequency slopes map the distance and radial velocity of target objects onto

the mean and the separation of two intermediate-frequency beat tones in a

delayed homodyne detection scheme. e, Schematic of the detected beat notes

arising in coherent lidar ranging of a moving object with optical carrier

frequency fopt. The ref lected laser light is both time-delayed and

frequency-shifted owing to the Doppler effect, leading to the observation of

two homodyne beat notes fbeat during one scan period of the laser.